This invention relates to a multiple wavelength light emitting device incorporated into with a single p-n junction which is capable of simultaneously emitting lights having different light emitting wavelengths from one another.
A multiple color light emitting device can emit a plurality of color lights simultaneously or each of them individually, and for reasons of this convenience it is used for photo-electric switches and the like. Recently, as an infrared light is increasingly employed in many applications, taking advantage of its invisibility, a demand for infrared light emitting devices made of compound semiconductor is also being increased.
Since a light emitted by such infrared light emitting device belongs to an infrared region or an invisible region, it is difficult to determine by view whether or not the device is malfunctioning, which gives rise to a problem in maintaining the performance of an electronic apparatus which employs such infrared light emitting device.
A compound semiconductor device which can emit an infrared light and a visible light is disclosed, for example, by Japanese Kokai No. 57-28371. This prior art proposes a multiple color light emitting device which comprises a first p-n junction formed of an n-type Ga.sub.x Al.sub.1-x As crystal layer and a p-type Ga.sub.x Al.sub.1-x As crystal layer epitaxially grown on a surface of an n-type GaAs crystal substrate, and a second p-n junction formed by diffusing impurities in the n-type Ga.sub.x Al.sub.1-x As crystal layer exposed by removing at least a portion of the n-type GaAs crystal substrate, wherein these two p-n junctions are utilized to emit the above-mentioned infrared light and visible light.
However, for manufacturing a device which can emit two kinds of lights, i.e., a red light and an infrared light on the basis of the multiple color light emitting device constructed as mentioned above, it is necessary to form two p-n junctions, in a Ga.sub.x Al.sub.1-x As crystal layer, oriented in opposite directions to one another by forming an epitaxially grown n-type Ga.sub.x Al.sub.1-x As crystal layer, diffusing Zn in the surface of the grown layer to form a p-type layer, and removing the GaAs crystal substrate to form another p-type layer on the back surface of the grown layer. Thus, the manufacturing process for such device having two p-n junctions is quite complicated. In addition, the prior art device requires adjustment of respective bias currents for red and infrared lights when they are simultaneously emitted, thereby making it difficult to employ a visible red light as a monitor for emission of an infrared light.
Another prior art example of a single light emitting device made of compound semiconductor which can emit an infrared light and a visible light is disclosed by Japanese Kokai No. 57-41979. This device makes use of the fact that the diffusion length of injected electrons from an n-region to a p-region in a light emitting diode made of GaAs doped with Si is sufficiently long. An n-type (doped with Si) GaAs mono-crystal layer, a p-type GaAs mono-crystal layer and a p-type GaAlAs mixed-crystal layer are grown on a GaAs substrate, wherein an infrared light is emitted from the GaAs mono-crystal layer, or a first p-type layer in a high current bias region, while a visible light determined by the forbidden band of the materials constituting the layer, for example, a red light is emitted from the GaAlAs mixed-crystal layer or a second p-type layer in the same manner.
However, the above light emitting device has a drawback in that electrons injected from the n-type layer almost recombine in the first p-type layer and therefore do not diffuse and recombine in the second p-type layer. Even if electrons recombine in the second p-type GaAlAs mixed-crystal layer to achieve light emission, the light is self-absorbed. Thus, its internal light emitting efficiency is quite low.